Remove experimental MSC2716 implementation to incrementally import history into existing rooms (#15748)
Context for why we're removing the implementation:
- https://github.com/matrix-org/matrix-spec-proposals/pull/2716#issuecomment-1487441010
- https://github.com/matrix-org/matrix-spec-proposals/pull/2716#issuecomment-1504262734
Anyone wanting to continue MSC2716, should also address these leftover tasks: https://github.com/matrix-org/synapse/issues/10737
Closes https://github.com/matrix-org/synapse/issues/10737 in the fact that it is not longer necessary to track those things.
0f02f0b4da